1. Field of the Invention
The present invention relates to a radiation treatment system designed to treat a tumor or the like in the body or the body surface of a patient by irradiating the tumor with particle beams such as a proton beam, a carbon beam or the like. More particularly, the invention relates to a radiation treatment system and a method thereof, capable of maintaining flatness, which indicates the degree of uniform radiation beam irradiation in an area to be exposed to radiation, by dividing the area to be exposed to radiation including its peripheral area into a plurality of areas, and then performing the simulation of radiation exposure according to the shape of each area, and improving the efficiency of radiation utilization.
2. Description of the Related Art
The radiotherapy is a method of treatment employed to reduce or even remove a tumor by intensively applying radiation to the tumor generated in the body of a patient. When such a radiation exposure treatment is carried out, the area to be exposed to radiation including the tumor to be treated must be uniformly irradiated with a proper dose of radiation. On the other hand, radiation applied to a healthy organization around the tumor outside the area to be exposed to radiation must be suppressed as much as possible.
Now, description will be made as to the radiation treatment system which uses particle beams among the radiations used for the above-described radiotherapy. Here, to briefly explain the principle of the radiation treatment system, consideration is given to a case where an area to be treated is a ball having a radius r, a patient is regarded as a cube having one side set equal to 4r, and the center of the ball as the area to be treated is located at the center of the cube.
FIG. 9 illustrates the configuration of a radiation treatment system using such particle beams available in the related art. In the drawing, a reference numeral 101 denotes a region to be treated by radiotherapy (hereinafter may be referred to just as treatment region), in which a tumor or the like to be subjected to radiation exposure is present. This region is assumed to be a ball having a radius r, and the center of the ball is located at a cubic center of a patient 102 assumed as a cube having one side set equal to 4r. A reference numeral 102 denotes a patient fixed to a treatment couch 114, and irradiated with particle beams; 103 a radiation exposure region defined in a three-dimensional area which includes a treatment region 101; 104 a treatment planning device designed to perform treatment simulation for forming a radiation exposure region 103 according to the state of the diseased part of the patient 102 to be treated, and setting parameters for the direction of irradiation, the position of irradiation, and so on; 105 an accelerator controller for controlling the operation of an accelerator 107, specifically designed to adjust a radiation beam to appropriate strength according to the acceleration condition in the treatment simulation performed by the treatment planning device 104; and 106 an irradiation controller designed to control a wobbler device 108, a scatterer device 109, a dose monitoring device 110, a ridge filter device 111, a range shifter device 112, and a collimator device 113 respectively according to a set condition in the treatment simulation, thereby controlling the irradiation direction, position, and so on, of the radiation beam.
A reference numeral 107 denotes an accelerator for offering energy to the radiation beam, a cyclotron, a synchrotron, or the like for accelerating the radiation beam containing charged particles by the acceleration electric field of high frequency being used therefor; 108 a wobbler device used to expand the radiation beam corresponding to the radiation exposure region 103, composed of a deflection electromagnet, and adapted to move the radiation beam so as to draw a circular orbit on the radiation exposure region 103 by applying a sinewave current having a phase different by 90° to this deflection electromagnet; 109 a scatterer device composed of a scatterer for scattering the radiation beam, and used to expand the radiation beam with the wobbler device 108 corresponding to the radiation exposure region 103; 110 a dose monitoring device for monitoring the dose of the radiation beam applied to the radiation exposure region 103, designed to output each dose of the radiation beam monitored to the irradiation controller 106; 111 a ridge filter device used for modulating the range of the radiation beam, generally made of brass or the like having a proper shape on the surface, and designed to adjust the expanse in the advancing direction of the radiation beam; 112 a range shifter device for adjusting a reaching distance in the advancing direction of the radiation beam according to the set condition in the treatment simulation; 113 a collimator device for adjusting the passage aperture of the radiation beam corresponding to the radiation exposure region 103; and 114 a treatment couch for laying the patient 102 thereon.
Next, the operation of the system as configured above will now be described as below.
First, before a particle beams treatment is carried out, the image data of a diseased part obtained by photographing the diseased part (equivalent to the treatment region 101) of the patient 102 with an X-ray CT not-shown is output to the treatment planning device 104. Based on the state of the diseased part analyzed from the input image data thereof, the treatment planning device 104 decides a radiation exposure region 103 by adding an area or the like as a margin to the treatment region 101, and then performs treatment simulation for setting parameters for the direction of irradiation, the position of irradiation, and so on.
In this case, in the radiation treatment system available in the related art, when the treatment region 101 is formed to be a ball having a radius r, a radiation exposure region 103 becomes a circle having a radius r. Then, the ridge filter device 111 having a width of 2r of SOBP (spread out Bragg peak) in the advancing direction of the radiation beam is used. Moreover, in the accelerator 107, the radiation beam is accelerated by acceleration energy that causes the reaching distance of particle beams in the body of the patient 102 to be 3r. Then, the treatment simulation is executed by the wobbling device 108 and the scatterer device 109 such that the radiation beam is uniformly applied in the radiation exposure region 103 (flattening).
Besides the foregoing, in order to accurately irradiate the radiation exposure region 103 with the radiation beam, there are cases where a collimator dedicated to the patient having a cylindrical radiation beam passage aperture is used, and the collimator device 113 having general applicability is used. FIG. 9 shows the example of using the collimator device 113.
Now, the treatment simulation will be described in detail. The treatment planning device 104 calculates a parameter to be set for the collimator device 113, assuming that the collimator is usually circumscribed on the circular radiation exposure region 103 having a radius r. Then, corresponding to the radiation exposure region 103, the treatment planning device 104 selects operation conditions for the wobbler device 108 and the scatterer device 109 for expanding the radiation beam. In addition, the ridge filter for causing the expanse in the advancing direction of the radiation beam to be 2r is selected.
In this case, if the reaching distance of the radiation beam is not exactly 3r in the body of the patient 102, then the treatment planning device 104 selects an operation condition for the accelerator 107 for offering radiation beam energy to realize a reaching distance of 3r or more. Subsequently, a difference in the reaching distance equivalent to the acceleration energy offered by the accelerator 107 and the reaching distance 3r of the radiation beam in the body of the patient 102 is adjusted by the range shifter device 112. The treatment planning device 104 decides an irradiation condition of the radiation beam to coincide with the deepest part in the radiation exposure region 103, and also decides the irradiation dose of particle beams at this time.
Checking is made as to the appropriateness of the dose distribution decided by the treatment planning device 104. If appropriate, then the treatment planning device 104 outputs the set parameter of the radiation treatment system which has been obtained in the treatment simulation, to the accelerator controller 105 and the irradiation controller 106. Upon having received the treatment parameter, the accelerator controller 105 and the irradiation controller 106 set the above-mentioned treatment parameter in each of the accelerator 107, the wobbler device 108, the scatterer device 109, the dose monitoring device 110, the ridge filter device 111, the range shifter device 112, and the collimator device 113.
Then, the patient 101 is laid on the treatment couch 114 and fixed, and the radiation exposure region 103 is aligned with the position of irradiation, and irradiated with particle beams. When the dose monitoring device 110 determines that the prescribed dose of radiation beam has been applied, the irradiation with the radiation beam is stopped, completing one treatment.
Next, description will be made as to a general method for performing flattening to uniformly irradiate the radiation exposure region 103 with the radiation beam, used in the particle-beams treatment. Hereinafter, the irradiation of the radiation exposure region 103 uniformly with the radiation beam is simply referred to as the flattening of a radiation field for the easiness of explanation.
In general, for the flattening of the radiation field, there are available a double scatterer system using a double scatterer, and a wobbler system using the wobbler device. These systems are both designed to form a circular radiation field (circular radiation exposure region 103).
More specifically, the double scatterer system increases the level of scattering the radiation beam by using two kinds of scatterers arranged away from each other in the axial direction of the radiation beam, and flattens the radiation field by performing scattering in such a way as to increase the efficiency of using the radiation beam. There is a close relation between the size of the radiation field (radiation exposure region 103), and the scattering conditions of the two kinds of scatterers and the shapes thereof. The efficiency of using particle beams in the double scatterer system is generally about 30%. The efficiency of using particle-beams is equivalent to the ratio of a dose of radiation applied in the radiation field with respect to all doses of radiation including a dose of radiation applied outside the radiation field for the flattening of the radiation field.
The wobbler system rotates the radiation beam by the wobbler device for generating a rotating magnetic field, scatters the radiation beam by the scatterer to expand a beam diameter, and then forms a radiation field having prescribed flatness (degree of uniformity when the radiation exposure region 103 is irradiated with the radiation beam, given by a difference in the reaching amounts of the radiation beams applied to the radiation exposure region 103) when the radiation beam makes one rotation. In this wobbler system, when the radiation field is enlarged, the rotational radius of the radiation beam is increased, and the thickness of the scatterer for expanding the radiation beam is increased.
There is a relation given by an equation below among the size (rmax) of the radiation field, the rotational diameter (R0) of the radiation beam, and the expanse (σa) of the radiation beam, when the flatness of the radiation field is ±2%. In this case, the radiation field is formed based on the characteristic of the radiation beam supplied from the accelerator according to a prescribed relational equation. In the wobbler system, since particle beams in the area of 84% inside the rotational radius of the radiation beam is used, the efficiency of using particle beams becomes about 30%.R0:σa:rmax=1.00:0.90:0.84Here, rmax in the above-mentioned relational equation becomes small when the flatness is further increased to reach ±1%, and the efficiency of using the particle beams is lowered.
The radiation treatment system of the related art is constructed in the foregoing manner, and the treatment simulation is performed for the circular radiation exposure region 103 formed by the double scatterer system or the wobbler system to decide parameters to be set for the respective devices. Consequently, the efficiency of using particle beams has been low.
To explain the foregoing problem more specifically, an actual radiation exposure region 103 is not always a circular radiation field. Thus, when the radiation field is subjected to flattening by the double scatterer system or the wobbler system, the proportion of particle beams applied outside the radiation exposure region 103 is increased, bringing about a reduction in the efficiency of using particle beams. For example, the efficiency of particle beams in the foregoing wobbler system was theoretically 30%. Actually, however, the efficiency is lower than this value, and if the radiation exposure region 103 occupies only ½ of the circular radiation field, then the efficiency of using particle beams is lowered to 15%.